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The forum as moved here : https://forum.babylonjs.com/t/welcome-to-the-new-babylon-js-forum/18 you should post there for any questions.
Regarding yours, the side effects not being filled in the package.json (currently under dev) you need to import from individual modules:
import { Engine } from "@babylonjs/core/Engines/engine";
import { Scene } from "@babylonjs/core/scene";
import { Vector3 } from "@babylonjs/core/Maths/math";
import { FreeCamera } from "@babylonjs/core/Cameras/freeCamera";
import { HemisphericLight } from "@babylonjs/core/Lights/hemisphericLight";
import { Mesh } from "@babylonjs/core/Meshes/mesh";
// Side effect import to fill in the default material on scene and Mesh.CreateBox functions
import "@babylonjs/core/Materials/standardMaterial";
import "@babylonjs/core/Meshes/Builders/boxBuilder";
This is all detailed in the package doc:
https://www.npmjs.com/package/@babylonjs/core/v/4.0.0-alpha.21
https://doc.babylonjs.com/features/es6_support#tree-shaking